Generally, an organic light-emitting phenomenon refers to the conversion of an electrical energy to light energy using an organic material. In other words, in a case where an organic material layer is positioned between a positive electrode (e.g. an anode) and a negative electrode (e.g. a cathode), when a voltage is applied between the two electrodes, holes from the anode and electrons from the cathode are injected into the organic material layer. When the injected holes combine with the injected electrons, excitons are formed. Then, when the excitons return to a ground state, light is generated.
For research on such an organic electroluminescent device, Bernanose in the 1950's applied a high AC voltage to a polymer thin film containing organic dye, and observed light emission from the organic thin film. Then, in 1965, he generated singlet excitons by applying current to anthracene single crystal, and thus obtained blue fluorescence.
As one method for efficiently fabricating an organic electroluminescent device, a research for fabricating an organic material layer in a multi-layer structure, instead of in a single layer structure, within the device, has been conducted. In 1987, Tang reported an organic electroluminescent device with a layered structure including function layers such as a hole layer and a light emitting layer. Most of currently used organic electroluminescent devices include a substrate, an anode, a hole injection layer receiving holes from the anode, a hole transport layer transporting holes, a light emitting layer emitting light through recombination of holes and electrons, an electron transport layer transporting electrons, an electron injection layer receiving electrons from a cathode, and the cathode. The reason the organic electroluminescent device is manufactured in a multi-layered structure is that a hole and an electron have different moving speeds. Thus, when hole injection/transport layers, and electron injection/transport layers are appropriately formed, holes and electrons can be effectively transported. This balances holes and electrons within a device, thereby increasing luminous efficiency.
The first report on a material for electron transport was on an oxadiazole derivative (PBD). Then, a triazole derivative (TAZ) and a phenanthroline derivative (BCP) were reported to show an electron transport capability. Also, it was reported that as an electron transport layer, organic metal complexes from among organic monomolecular materials, which have a high stability against electrons and showing a relatively high electron moving speed, are preferable. Especially, Alq3 having a high stability and a high electron affinity was reported to be the most excellent, and is currently generally used. Also, there are conventionally known electron transport materials such as a flavon derivative (Sanyo), or germanium and silicon cyclopentadienone derivatives (Chisso) (Japanese Patent Publication Nos. 1998-017860, and 1999-087067).
Also, as materials for electron injection/transport layers, organic monomolecular materials having an imidazole group, an oxazole group, and a thiazole group have conventionally frequently been reported. However, before these materials were reported as the electron transport materials, the application of the materials' metal complex compounds to a blue light emitting layer or a blue-green light emitting layer of an organic light emitting device had been already reported in EU 0700917 A2 (Motorola).
TPBI, which was reported from Kodak in 1996 and disclosed in U.S. Pat. No. 5,645,948, is known to be a representative electron transport layer material having an imidazole group. Structurally, it contains three N-phenyl benzimidazole groups at 1,3,5 substitution positions of benzene. Also, functionally, it can not only transport electrons but also block holes from a light emitting layer. However, there is a problem in that TPBI has a low thermal stability for actual application to a device.
Also, there are other electron transport materials disclosed in Japanese Patent Publication Hei 11-345686, which were reported to contain an oxazole group, and a thiazole group, and to be capable of being applied to a light emitting layer. However, they are not yet put to practical use in view of driving voltage, luminance, and device lifetime.
Accordingly, in order to overcome the above described problems of a conventional technology, and to further improve the characteristics of an organic electroluminescent device, it is continuously required to develop a more stable and more efficient material capable of being used as an electron transport material in the organic electroluminescent device.